I must have too much time on my hands today. I have been wondering where in the firing cycle the max pressure of a load is reached and how far down the barrel the bullet might be by that point. Revolvers have a certain amount of venting built in, but a rifle doesn't. Might be unanswerable since the time frame is in microseconds, but do any of you experts have any info on this? Goatwhiskers

First, this was a pretty poor load, giving a huge extreme spread of pressures. But even in this high expansion ratio case (.20-caliber with 35 grains of water capacity case) the pressure consistently peaked about a third of the way down the 24" barrel. In a case with a lower expansion ration, it would occur even closer to the breech.

Also note that at bullet exit (the small crosses) there was still about 20K of pressure in the barrel and that the pressure does NOT drop instantly to zero. That's due to the mass of the gas in the bore; it can't get out of its own way fast enough to instantly vent to the atmosphere.

That's true. Most 9mmP loads probably peak before the bullet clears the case mouth. The 357 Mag and 30 Carbine (revolver) peak just about the time the bullet clears the cylinder gap - which is why they are ear-bleed loud.

In your Pressure Trace plot firing, the bullet is not a third of the way down the barrel at the pressure peak. It is simply a third of the way through the total barrel time. Because the bullet starts out much slower at the breech end than it gets to at the muzzle, it doesn't cover nearly as much distance per microsecond at the breech as it does at the muzzle. The result is a lot more of the barrel time is spent getting over the first inch of barrel than the last.

To illustrate this, I used pressure trace data I took on my dad's 03A3. The plot is the first one, below. (Plot T1 is not displayed because it had an anomaly; never dropped back near zero after the bullet exited.) This was for well-aged (1977) Lake City M2 ball ammo loaded with 56.1 grains (measured average from pulled rounds) of WC852 under a 152 grain flat base FMJ bullet. The bullet pull is highly irregular on this old stuff, so the pressure curves aren't overly consistent.

Below that trace is an Excel plot. What I did was open the pressure trace file in Notepad and extract the pressure data for the four traces and put it into Excel. The data is in 10 microsecond (0.01 millisecond) steps. I then averaged all four traces into one combined pressure trace (dark blue line), converted the psi to pounds force on the bullet base based, subtracted a friction allowance, then divided it by the mass of the bullet and multiplied by ten microseconds to get the velocity gain at the end of each step. I then added the result to the velocity at the end of the previous step. In another column I used the average velocity in the middle of each step to determine how many inches of bullet travel occurred during the step. The result is the yellow line, which is bullet progress down the barrel over time. If you follow the intersect of the bullet travel with the pressure peak, you'll see it has only traveled about 1.4 inches when the peak occurs.

"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity." Homer Powley

You are absolutely right, Nick. My brain wanted to type "before the bullet gets a third of the way down the barrel" but my fingers went ahead and typed "about a third." Since I had no way to tell from the trace exactly where the peak occurred (in inches of bullet travel) I chickened out - or my fingers did.

If I were to guess, I'd say that it occurs from four to six inches down the bore, but that's the best I can do. The greater distance compared to the .30-06 load you profiled would be due to the much lower expansion ratio of the .20-cal round. Regardless of where the peak occurs in a firearm, that's the point at which the bulk of the powder has burned and whatever remains can no longer produce enough gas to overcome the rapidly increasing "chamber" volume created by the moving bullet.

The yellow line on the graph (if you can make it out) shows 1.4 inches of bullet travel at the pressure peak. I ran the same load, with some adjustment, using H380 (canister grade WC852) in QuickLOAD and, despite a slightly different shape to the pressure rise portion of the curve, it also came up with about 1.4" down the bore at peak. If I change some assumptions I can get that to move forward to about 1.6" using the Pressure Trace data, but that seems to be about all the wiggle room there is.

The peak happens just about or slightly after the point where the bulk of the powder has finished its progressive burning phase and has switched over to its digressive burning phase. That's where the walls between perforations are burned through in stick powders so surface area of the burn starts to shrink rather than grow, or else is where a spherical propellant runs out of deterrent coating, or a combination of the two in some powders. The bullet gets going faster there plus the amount of new heat being generated by the digressive burn smaller per second, so that phase of the burn can take some time to complete. Often, by muzzle exit, it's not quite done.

At 1.4" bullet travel, the base of the M2 ball has just cleared the military throat and its bearing surface extends about a half inch deeper into the bore. The other thing that tells you that's where the bullet was at peak pressure is just in front of the throat is where the metal fouling builds fastest. When the pressure difference from the front to the back of the bullet is greatest, the bullet has the greatest upsetting force perpendicular to the bore axis, so friction between it and the bore are greatest there and remains pretty high for another couple of inches of travel before starting to back off as the pressure declines.

"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity." Homer Powley

20K is too high for the muzzle, but 2-3kpsi is too low. I say that because it is lower than any estimate or measurement I've seen of the pressure needed to overcome jacketed bullet friction in the bore. If your pressure numbers were correct, due to that friction, high power rifle bullets would slow with each additional inch of barrel rather than gain 20-40 fps or so per inch, as they are measured to do.

Remember that the gauge in Rocky's plot is at the chamber end of the rifle. There is a pressure gradient on the order of several kpsi between the breech pressure and the base of the bullet scooting down the bore. So we always expect the breech end reading to be higher at the time of bullet exit than the actual muzzle pressure is.

"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity." Homer Powley